Plasma etch and deposition processes have become the dominant pattern transfer means used in semiconductor manufacturing over the past 20 years. Most plasma based processes employ the fundamental principle of disassociation of a feed gas by the application of radio frequency (RF) power. As with all plasma loads, one of the dominant characteristics of the plasma load is its non-linearity. The non-linearity of the load affects the voltage and current sine waves of the delivered RF power by creating prevalent harmonic distortion. The exact amount of harmonic distortion, as represented by the amplitude of the harmonic frequencies and the associated phase angle of the current harmonic relative to the corresponding voltage harmonic, is unique to the plasma creating them. To be more precise, the plasma parameters, including ion and electron densities and energies, collision frequencies, neutral constituents, and their respective densities all contribute in a unique way to the amplitude of specific harmonic components of the fundamental frequency applied by a power delivery source to achieve the desired disassociation and subsequent process results.
It is thus apparent that, by monitoring the harmonic components of the fundamental frequency applied by a power delivery source, enhanced process control of plasma deposition and etch processes may be obtained. Consequently, several products have been developed that are designed to provide enhanced process control by monitoring such RF harmonic content. Unfortunately, wide scale proliferation of this technology has not been realized due to several fundamental limitations in the available technology.
One of the most significant limitations in the existing technology has to do with product architecture. Existing products typically contain a transducer package, commonly located at the point of measurement, and a corresponding analysis, control and communications package, which is typically located remotely from the point of measurement. Since each transducer package provides a unique output, these two packages are specifically calibrated to work with each other. Consequently, it is not possible to replace either package independently of the other without recalibrating the system. Since downtime is extremely expensive in a semiconductor processing line, this deficiency creates fatal maintenance and support issues for users of these RF sensor based process control solutions.
Although several devices are known for monitoring the harmonic content of delivered RF power, each requires precise calibration of individual components. Original hardware designed for plasma process control RF sensors in existing solutions has been based on either: a) RF switch routed band pass filters; (b) directional couplers; or (c) heterodyne or digital signal processor circuitry enabled with programmable local oscillators. Each of these designs comprises a transducer package with corresponding analysis and communications electronics package. In each case, the design is not capable of having a replacement substituted for either component package without the necessity to recalibrate the entire RF sensor device (consisting of both the transducer and the analysis/communication packages).
Consequently, a need exists in the art for methods and devices that will support a field replacement strategy that allows any transducer package to function properly with any corresponding analysis and communications package, without degradation in performance and without the need for recalibration.
Another issue with existing devices for monitoring the harmonic content of delivered RF power has to do with self-bias voltage. The RF power in plasma etch reactors is typically delivered to a capacitively coupled electrode. The capacitive coupling halts the flow of DC current in the direction of the RF power delivery network creating a “self-bias” voltage on the electrode surface. This “self-bias” voltage is always negative and may aid in the etch process by accelerating plasma ions in the direction of the substrate surface, thus providing much needed activation energy for the volatilization or polymerization process.
In light of its beneficial effects, it is desirable to know the self-bias voltage of a system, along with the components of the delivered RF power. However, although several voltage sampling schemes have been proposed in the art for monitoring the harmonic content of delivered RF power, to date, none of these schemes also allow the self-bias voltage to be monitored. U.S. Pat. No. 5,867,020 (Moore et al.), which discloses a commonly used capacitively coupled RF voltage probe, is exemplary. There is thus a need in the art for a method for monitoring the harmonic content of delivered RF power in a way that also allows the self-bias voltage to be monitored.
A further issue with existing devices for monitoring the harmonic content of delivered RF power concerns the shielding of the inductive transducer. Due to the pressure-flow regime and the molecular stability of many gases used in semiconductor processing, often a relatively high RF voltage is required to initialize and sustain the process plasma. In addition, the diode-like characteristics of plasmas can cause the RF current flow after ignition to be very high. The RF power is typically delivered to a capacitively coupled electrode where the flow of DC current is blocked, thus resulting in the DC “self-bias” voltage. Consequently, there is no need for a DC coupled RF current transducer to monitor the current component of the delivered RF power. However, there is a vital need to protect the simple inductive monitoring device, which operates in accordance with Faraday's Law, from stray fields (both magnetic as well as electric) that can significantly impact the potential for accurate RF current measurements.
Boundary condition analysis indicates that a grounded shield must be placed between the inductive transducer and the RF current carrier in order to properly shield the inductive transducer from the electric field radiating from the primary RF current carrier, and to thereby avoid crosstalk between voltage and current. Moreover, in order to shield the inductive transducer from stray electric and magnetic fields which may be in the ambient environment local to the measurement (such as the coil of an impedance matching network), the inductive transducer should be enclosed in a grounded shield. Unfortunately, the use of conventional shields to protect the transducer from ambient stray fields also impedes the measurement of the desired primary RF current magnetic field.
Various shield designs have been proposed in the art. However, none of these designs overcome the above noted infirmity. Thus, for example, U.S. Pat. No. 5,808,415 (Hopkins) and U.S. Pat. No. 6,061,006 (Hopkins) teach a dual loop antenna approach for monitoring RF current. U.S. Pat. No. 6,501,285 (Hopkins et al.) teaches an approach to assembling the inductor using individual printed circuit boards interconnected with metal filled vias to provide connection between the respective layers. U.S. Pat. No. 5,834,931 (Moore et al.) teaches a single turn first principles implementation of Faraday's law which, unfortunately, is limited by the propensity for arcing between the primary RF current carrier and the shield of the inductive loop.
There is thus a need in the art for a means to protect inductive monitoring devices which operate in accordance with Faraday's Law from stray fields (both magnetic as well as electric) that can significantly impact the potential for accurate RF current measurements. There is also a need in the art for such a device that does not impede the measurement of the desired primary RF current magnetic field.
A further issue related to the monitoring of the harmonic content of RF power sources concerns end point detection. Chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD) processes have become a vital component of semiconductor manufacturing over the past 20 years. CVD and PECVD processes are commonly used to deposit dielectric films at low temperatures to serve as either sacrificial layers or between metal layers as dielectric separation.
A non-value added, but essential, process step associated with both CVD and PECVD involves the plasma based cleaning of the chamber and associated components to remove residual film left after the deposition process. During the deposition process, the film is intentionally deposited on the semiconductor substrate. Chamber cleans are performed after the semiconductor substrate has been removed from the chamber, and as such, are critical to the success of the deposition process but are not actually a part of semiconductor device fabrication. The common means for chamber clean steps is plasma based volatilization of the deposited film.
A fundamental principle employed in most plasma based processes is disassociation of a feed gas by the application of radio frequency (RF) power. As a non-value added event, it is vital to minimize the duration of the chamber clean. Also, it has been documented that prolonged cleaning can actually degrade chamber components, thus resulting in the creation of yield limiting particles. Hence, in order to minimize manufacturing costs while maximizing step yields, it is imperative to know when to stop the clean process. The correct moment at which to halt the clean process is called end point.
RF end point detection is based on monitoring the components of the delivered RF power. As the film clears from the chamber components, the by-products of the volatilized film volumetrically decrease in the plasma. This volumetric change in the plasma components creates an impedance change seen by the RF power supply network, and results in consequential changes in the RF voltage, current, phase angle and self-bias voltage. By monitoring the changes in these signals, a correct determination of the RF end point may be obtained. Significantly, it is not necessary that the film type, film thickness or pattern density be consistent from run to run in order for the detector to properly function, since the signal analysis algorithm will be the compensating factor.
Various devices have been designed for monitoring the components of delivered RF power in semiconductor processing. Such devices are discussed, for example, in U.S. Pat. No. 5,770,992 (Waters), U.S. Pat. No. 5,565,737 (Keane), U.S. Pat. No. 6,046,594 (Mavretic), U.S. Pat. No. 5,808,415 (Hopkins) and U.S. Pat. No. 6,061,006 (Hopkins). All of these devices rely on AC coupled voltage and current measurements of the delivered RF power which serve as input signals to frequency discriminating detection circuits for harmonic analysis. Such a configuration places limitations on the detector circuits that can be used to analyze the broadband, harmonically distorted RF signals. Moreover, these devices require interface electronics to process the sampled signals before use in any subsequent application. Also, each of these devices is configured such that the transducer package and associated analysis or interface electronics package are calibrated together and cannot be separated without failure or degradation in overall performance. The shortcomings of such a configuration have been discussed above. There is thus a need in the art for a device for monitoring the harmonic content of delivered RF power that overcomes these deficiencies.
Another issue relating to RF power supplies for plasma reactors concerns the diagnosis of the components of an RF power delivery network. Semiconductor manufacturing facilities are extremely expensive to construct and operate. Consequently, every effort is made to minimize manufacturing tool down time, and maintenance and recovery of an off-line tool is always under excessive time constraints. Often, when a tool is taken off-line due to a failure to meet performance specifications, repair efforts suffer from a lack of diagnostics. Consequently, such repair efforts often become extremely expensive.
FIG. 14 shows a typical configuration for such a system. The system 100 comprises an RF generator 101, an impedance matching network 130, and a load 150. Generator 100 is coupled to impedance matching network 130 through a known impedance 120. This impedance is typically a nominal characteristic value, such as 50 ohms. Impedance 120 serves to facilitate optimal power transfer from the generator to matching network 130. The impedance 140 seen between matching network 130 and load 150 is generally unknown and varies over time.
Most RF power generators have “built-in” output measurement capability, but this is typically located remote from the impedance matching network. Measurement of input power at the input of the impedance matching network has historically been provided by utilization of bolo-meters, calorimeters, diodes and other types of instrumentation. Examples of prior art methods for making RF power measurements in coaxial environments may be found in U.S. Pat. No. 4,547,728 (Mecklenburg), U.S. Pat. No. 4,263,653 (Mecklenburg) and U.S. Pat. No. 4,080,566 (Mecklenburg), all of which rely on an inductive coil design to sample the RF voltage. However, since the measurement to be performed is typically diagnostic and only necessary during maintenance and troubleshooting, the cost, portability and ease of installation are of paramount concern.
Typical prior art methods for measuring the power at the output of the impedance network rely on alternating current (AC) coupled voltage and current measurements of the RF power delivered to the load. These measurements are input to frequency discrimination circuitry for the purpose of performing harmonic analysis. Examples of prior art methods for monitoring components for delivering RF power in semi-conductor processing are described in numerous patents, including, for example, U.S. Pat. No. 5,770,992 (Waters), U.S. Pat. No. 5,565,737 (Keane), U.S. Pat. No. 6,046,594 (Mavretic), U.S. Pat. No. 5,808,415 (Hopkins) and U.S. Pat. No. 6,061,006 (Hopkins). These systems also comprise a transducer package and associated analysis or interface electronics package which are calibrated together, and thus have the infirmities noted above (that is, they cannot be separated without degradation in overall performance).
Traditional RF power measurement technologies offer solutions in either the characteristic impedance portion of the delivery network or the non-characteristic impedance section without any integration of the two measurement devices. Some attempts have been made to integrate expensive and difficult to install frequency discriminating RF sensors, but these have met with poor acceptance due to price and installation issues. There is thus a need in the art for a means for field engineers to quickly, easily, cheaply and accurately diagnose the components of the RF power delivery network and determine which, if any, components of the system are faulty.
The above noted needs are met by the devices and methodologies disclosed herein and hereinafter described.